BMW’s Brake Energy Regeneration system is a system that only charges the battery when the car is coasting, braking, or decelerating. During acceleration and whenever possible during other driving the alternator is decoupled from the drivetrain, leading to better fuel economy and improved performance. Conventional alternators charge your battery whenever the engine is running and the engine’s serpentine belt is turning. The BMW Brake Energy Regeneration system charges the battery at times when the kinetic energy would otherwise be wasted (usually converted into heat by the brakes), increasing fuel efficiency by up to 3%.
By decoupling the system during acceleration all of the engine’s power can be sent to the wheels. The system continuously monitors the battery’s state of charge, and can, if necessary, charge the battery even when accelerating in order to prevent it from becoming discharged.
The Brake Energy Regeneration system is also used on ActiveHybrid hybrids and ActiveE fully electric vehicles. On these vehicles the motor that powers the wheels turns into a generator as soon as the accelerator is released, increasing their range by up to 20%.